1. Field of the Invention
The present invention relates to an image transfer apparatus utilizing an image carrier such as a dielectric material drum, photosensitive material drum and photosensitive material belt, etc. or an image transfer member for transferring a visible image on an intermediate transfer body to a recording medium or a simultaneously transferring and fixing member, an electrophotographic copying apparatus for obtaining an image with one or a plurality of image forming means and an image forming apparatus such as electrophotographic printer, ionographic printer, ink jet printer and facsimile apparatus, etc. and particularly to a transfer apparatus and image forming apparatus which controls speed variation of an image carrier and intermediate transfer body.
2. Description of the Related Art
As a transfer apparatus of related art, for example, an apparatus utilizing a bias transfer roll is popular. In this transfer apparatus, a bias transfer roll is pressed toward to a photosensitive drum, on which a toner image is formed while it is rotating at the predetermined speed, at the predetermined contact pressure with a spring and a toner image on the photosensitive drum is transferred to a recording paper while the paper passes the transfer section (nip section) formed between the photosensitive drum and bias transfer roll. According to this transfer apparatus, the recording paper itself is charged in a small extent, unlike the corona transfer method, and therefore the recording paper can be separated easily from the photosensitive drum and excellent transfer image can be obtained without generation of any irregular image due to the separation discharge at the time of separation of paper.
Moreover, an image forming apparatus of the related art is described, for example, in the Japanese Published Unexamined Patent Application No. Hei 6-27832 and Japanese Published Unexamined Patent Application No. Hei 6-317994.
The image forming apparatus proposed in the Japanese Published Unexamined Patent Application Hei 6-27832 comprises a photosensitive drum for carrying a toner image, an intermediate transfer and fixing belt extended over a heat roll, a drive roll and a transfer roll and a spring which energizes the transfer roll in the direction to be isolated from the photosensitive drum. In this structure, when the drive roll is driven to rotate, the transfer roll is displaced with a tension of the intermediate transferring and fixing belt, causing the intermediate transferring and fixing belt to be brought in pressure contact with the photosensitive drum. Thereby, while the copying operation is stopped, the intermediate transferring and fixing belt is isolated from the photosensitive drum to prevent the toner on the photosensitive drum to be adhered to the intermediate transferring and fixing belt to contaminate it.
The image forming apparatus proposed in the Japanese Published Unexamined Patent Application No. Hei 6-317994 is provided with a resist sensor for detecting front end and rear end of the recording paper and a cam for causing the transfer roll to be in contact with or separated from the image carrier without giving any impact thereto depending on detection by resist sensor. In this structure, when the resist sensor detects the front end of the recording paper, the transfer roll is brought in pressure contact with the image carrier due to rotation of the cam to transfer the toner image on the image carrier to the recording paper until the front end of the recording paper reaches the transfer area between the image carrier and transfer roll. When image transfer progresses and the resist sensor detects the rear end of the recording paper, the transfer roll is isolated from the image carrier due to rotation of the cam when the rear end of paper passes the transfer area. Thereby, since the transfer roll is isolated from the image carrier when the recording paper does not exist, the toner remaining on the image carrier is transferred to the transfer roll to prevent contamination of back surface of the subsequent recording paper. Moreover, when the transfer roll is placed in contact with the image carrier, no impact is given owing to the cam and thereby disturbance of image to be written to the image carrier can be prevented.
However, according to the transfer apparatus of the related art, when the recording paper passes the area between the photosensitive drum and transfer roll, particularly when the front end of paper enters the nip section between the photosensitive drum and transfer roll and the rear end of paper leaves the nip section, an impact is given to the photosensitive drum and transfer roll due to a level difference resulting from thickness of recording paper and thereby a load torque of the photosensitive drum varies, resulting in a problem that a rotating speed of the photosensitive drum varies. Particularly, this problem is serious for running of a thick recording paper.
Moreover, in the structure that distance between the transfer process position and fixing process position is shorter than the length of recording paper and thereby the paper is carried by both processes, the paper speed in the fixing process is generally a little lower than the paper speed in the transfer process to generate a certain looseness in order to absorb an error of speed in both processes. However, in this case, the recording paper is further carried and a loop of the paper grows, allowing an internal stress accumulated in the paper to increase. As a result, a stress of the paper gives a resistance force in the rotating direction of the photosensitive drum and transfer roll. Therefore, when the rear end of the paper is released from the transfer roll, such resistance force is also released, resulting in a large variation of rotation. This problem also exists in the monochromatic printer and it is a serious problem of image quality such as displacement of color and uneven color tone in the full-color printer.
Moreover, according to an image forming apparatus of the related art described in the Japanese Published Unexamined Patent Application No. Hei 6-27832, the intermediate transferring and fixing belt is selectively used for transfer operation to the photosensitive drum and other operations, and transitional impact has never been alleviated when the recording paper enters the nip section or leaves the nip section. Therefore, the paper gives a transitional impact to the nip section it passes therein to give transitional speed variation to the image carrier, resulting in a defect in the image quality such as displacement of color and uneven color tone.
Moreover, according to the image forming apparatus described in the Japanese Published Unexamined Patent Application No. Hei 6-317994, certain consideration is taken into account in order to eliminate an impact generated when the transfer roll is placed in contact with the photosensitive drum, but the transitional impact generated when the recording paper passes through the nip section is not alleviated, similarly resulting in a defect in image quality such as displacement of color and uneven color tone.